Bone fixation method

ABSTRACT

A bone fixation assembly for securing a fixation device such as a bone plate, to bone includes a fixation device including a through hole; a bushing having a sidewall with a bore and a slot; a fastening screw with a threaded shaft and a head including a recess; and a locking screw with a body capable of being threaded into the head of the fastening screw to expand its radial wall. In use, the bushing is seated within the through hole of the fixation device at a selected angle and the fastening screw is inserted through the bore of the bushing such that its head fits in the bushing and its shaft is threaded into bone. The locking screw may then be threaded into the head of the fastening screw to expand the radial wall of its head resulting in the locking of the fastening screw, and bushing at a selected orientation within the through hole.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of prior patent application Ser. No.09/453,911, filed Apr. 19, 2000, now U.S. Pat. No. 6,235,033. The entirecontent of this application is expressly incorporated herein byreference thereto.

FIELD OF THE INVENTION

The present invention is directed to a bone fixation assembly, and inparticular to a bushing and screw assembly for securing a fixationdevice to bone.

BACKGROUND OF THE INVENTION

Orthopaedic fixation devices, both internal and external, are frequentlycoupled to bone by the use of fasteners such as screws or pins. Forexample, bone plates can be secured to bone with bone screws insertedthrough plate holes. Securing the screws to the plate provides a fixedangle relationship between the plate and screw and reduces the incidenceof loosening. One method of securing the screw to the plate involves theuse of so-called “expansion-head screws.” U.S. Pat. No. 4,484,570discloses an expansion-head screw with a head that has a recess, thewalls of which contain a number of slits. After the expansion-head screwis inserted into bone through a hole in the fixation device, a lockingscrew is inserted into the recess to expand the walls of the recess tothereby lock the screw to the fixation device.

In addition to securing the screw to the fixation device, it is alsooften desirable to insert the screws at an angle relative to thefixation device selected by the surgeon. The prior art discloses anumber of these so-called “polyaxial” systems, most of which utilize abushing located in a hole in the fixation device to provide forangulation of the screw relative to the fixation device. For example,U.S. Pat. No. 5,954,722 discloses a polyaxial locking plate thatincludes a plate hole having a bushing rotatable within the hole. As ascrew is being inserted into bone through the bushing and plate hole, athreaded head of the screw engages a threaded internal surface of thebushing to expand the bushing against the wall of the plate hole,thereby locking the screw at the desired angular orientation withrespect to the plate.

The present invention relates to an improved bone fixation assembly thatallows for a surgeon-selected angle of a fastening screw relative to thefixation device and locking of the fastening screw once the screw is atthe desired orientation.

SUMMARY OF THE INVENTION

The present invention relates to a bone fixation assembly for securing afixation device, such as a bone plate, to bone. The assembly includes afixation device, a bushing, a fastening screw, and a locking screw. Thebushing has a sidewall with an exterior surface configured anddimensioned for polyaxial rotation within the through hole of thefixation device and an interior surface which defines a bore thatextends through the upper and lower surfaces of the bushing. Thesidewall has at least one slot for allowing outward expansion of thesidewall against the through hole of the fixation device to thereby lockthe bushing at a selected angle relative to the axis of the through holeof the fixation device.

The fastening screw has a threaded shaft configured and dimensioned forinsertion through the bore of the bushing and threadably insertable intobone to secure the bushing and fixation device to bone. The head of thefastening screw is configured and dimensioned for insertion in thebushing and includes a radial wall and open end defining a recess. Theradial side wall has at least one slit for allowing outward expansion ofthe radial wall thereby outwardly expanding the sidewall of the bushing.The locking screw has a threaded body threadably received in the head ofthe fastening screw to thereby outwardly expand the radial wall of thefastening screw.

The bushing can have a substantially frustospherical shape with the boreextending through the central axis of the bushing, perpendicular to theupper and lower surfaces. In an exemplary embodiment, the bushing has anannular shoulder and the head of the fastening screw is flush with theupper surface of the bushing when the head of the fastening screw restsagainst the shoulder. The fastening screw can be provided with a lipprotruding from the radial wall which rests on the annular shoulder whenthe fastening screw is threaded into the bone.

In one embodiment, the sidewall of the bushing includes a plurality ofslots extending from the upper surface of the bushing. Furthermore, oneof the slots can extend from the upper surface completely through thelower surface. In order to further increase the security of the bushingwithin the through hole, the exterior surface of the sidewall of thebushing and/or the walls of the through hole of the fixation device canbe provided with ridges.

In order to eliminate the need to tap a drilled hole, the fasteningscrew can be a self-tapping screw. The fastening screw could also be aself-drilling screw. In an exemplary embodiment, the head of thefastening screw includes a tapered portion proximal to the threadedshaft.

In one embodiment, the recess of the fastening screw has a bottomsurface with a threaded hole for threadably receiving at least a portionof the threaded body of the locking screw. The locking screw can have atapered end. Additionally, the radial wall of the head of the fasteningscrew can have a matching tapered inner surface.

The present invention also relates to a method for securing a fixationdevice having a through hole to bone. A fastening screw is insertedthrough a bushing located in the through hole of a fixation device. Thefastening screw is then threaded into a bone. After the threading stepis completed, the fastening screw is locked to the bushing and fixationdevice. The fastening screw can be inserted through the bushing at asurgeon-selected orientation relative to the fixation device. Thefastening screw head is preferably expanded against the bushing tothereby lock the bushing to the fixation device. A locking screw can beinserted into a recess in the fastening screw head to cause theexpansion.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the present invention are disclosed in theaccompanying drawings, wherein similar reference characters denotesimilar elements throughout the several views, and wherein:

FIG. 1 shows a top view of a bone plate with four plate holes, eachplate hole having a bushing according to the present invention.

FIG. 2 shows a sectional view through line 1—1 of FIG. 1.

FIG. 3 shows a top view of the bushing of FIGS. 1 and 2.

FIG. 4 shows a sectional view through line 3—3 of FIG. 3.

FIG. 5 shows a perspective view of the bushing of FIGS. 1-4.

FIG. 6 shows a perspective view of a bone fixation assembly according tothe present invention with a portion of the fastening screw shown incross section to illustrate insertion of the locking screw.

FIG. 7 shows a top view of the fastening screw of FIG. 6 prior toinsertion of the locking screw.

FIG. 8 shows a top view of the locking screw of FIG. 6.

FIG. 9 shows a sectional view of a bone plate having a second embodimentof the bushing according to the present invention.

FIG. 10 shows a section view with a drill inserted in one of the plateholes.

FIG. 11 shows a sectional view with the fastening screws being insertedinto the bushings.

FIG. 12 shows a sectional view with the locking screws inserted into thefastening screws.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For convenience, the same or equivalent elements in the variousembodiments of the invention illustrated in the drawings have beenidentified with the same reference numerals. Further, in the descriptionthat follows, any reference to either orientation or direction isintended primarily for the convenience of description and is notintended in any way to limit the scope of the present invention thereto.

FIGS. 1 and 2 show a bone fixation device 10 having through holes 12 forreceiving a fastener to secure device 10 to bone. Although device 10 isshown as a bone plate, device 10 can be any suitable internal orexternal fixation device. Examples of such devices include an externalfixator, a clamp for a spinal rod, and the like. Each of through holes12 has a bushing 14.

As best seen in FIGS. 3-5, bushing 14 has an upper surface 16, a lowersurface 18, and a sidewall 20. Sidewall 20 has an exterior surface 22configured and dimensioned for polyaxial rotation within through hole12. As a result and as described in more detail below, a fastenerinserted through a bore 24, which is defined by an interior surface 26of bushing 14 and extends through both upper and lower surfaces 16, 18,can be inserted at a wide variety of orientations relative to device 10.In an exemplary embodiment, bushing 14 has a frustospherical shape.Alternatively, bushing 14 can have a frustoconical shape. With eithershape, bore 24 can extend through the central longitudinal axisperpendicular to the parallel upper and lower surfaces 16, 18.

Bushing 14 includes slots 28 located on sidewall 20. Slots 28 allowsidewall 20 to expand outwardly against through hole 12. This outwardexpansion locks bushing 14 at the selected orientation relative to theaxis of through hole 12. In order to enhance the locking effect uponexpansion, exterior surface 22 of sidewall 20 and/or the periphery ofthrough holes 12 can be provided with ridges 30. Ridges 30 provide anadditional mechanism to resist motion of bushing 14 relative to device10 once sidewall 20 has expanded outwardly. Although bushing 14 is shownhaving four slots, any number of slots, including one, can be used aslong as the chosen number of slots provides for outward expansion ofsidewall 20. As shown in FIG. 5, slot 28 a extends from upper surface 16through lower surface 18 while the rest of slots 28 do not extendthrough to lower surface 18. Slots 28 all extend from upper surface 16of bushing 14. However, as shown in FIG. 9, slots 28 can extend fromboth upper and lower surfaces 16, 18. Although slots 28 of FIG. 9 extendin an alternating pattern, other patterns can be used.

As shown in FIG. 6, the bone fixation assembly according to the presentinvention also includes a fastening screw 32 and a locking screw 34.Fastening screw 32 has a threaded shaft 36 that inserts through bore 24of bushing 14 and is threaded into bone to secure bushing 14 and device10 to the bone. Fastening screw 32 can be self-tapping to eliminate theneed to pre-thread the hole made in the bone. Further, fastening screw32 can be self-drilling to also eliminate the need to pre-drill thehole. A head 38 of fastening screw 32 substantially sits within bushing14 when fastening screw 32 has been threaded into the bone. In anexemplary embodiment, bore 24 has an annular shoulder 40 (FIG. 4) sothat head 38 is flush with upper surface 16 of bushing 14 when head 38rests against shoulder 40. This results in a low profile for thefixation assembly according to the present invention. Head 38 can beprovided with a lip 42 protruding from radial wall 44. Lip 42 restsagainst shoulder 40 when fastening screw 32 has been threaded into thebone to further reduce the profile.

Radial wall 44 includes at least one slit 46 to enable outward expansionof radial wall 44. Thus, when head 38 is set withing bore 24 of bushing14, outward expansion of radial wall 44 of fastening screw 32 causesoutward expansion of sidewall 20 of bushing 14. Fastening screw 32 caninclude a tapered portion 48 proximal threaded shaft 36 to maximize theexpanding effect as fastening screw 32 is threaded into bone throughbore 24 of bushing 14.

As head 38 of fastening screw 32 is open-ended at its top, the open endand radial wall 44 define a recess 50 into which locking screw 34 isinserted to outwardly expand radial wall 44. Locking screw 34 has athreaded body 52 that is received in a threaded blind hole 54 infastening screw 32. Radial wall 44 of head 38 fastening screw 32 has atapered inner surface 56 that cooperates with tapered end 58 of lockingscrew 34 so that as locking screw 34 is threaded into blind hole 54,tapered end 58 urges radial wall 44 outward.

In use, bone fixation device 10 is placed against bone (FIG. 9). Iffastening screw is not self-drilling, a hole is drilled through bone atthe desired orientation with respect to device 10 (FIG. 10). This isachieved by rotating bushing 14 within through hole 12. Fastening screw32 is inserted through bushing 14 and threaded into bone to securedevice 10 to bone (FIG. 11). Head 38 can be provided with a mechanismfor mating with a driver for threading into bone. For example, FIG. 7shows that slots 28 are formed in a pattern that allows slots 28 toreceive a blade of a driver for insertion into bone.

After fastening screw 32 is threaded into bone, locking screw 34 is theninserted into recess 50. As locking screw 34 is threaded into threadedblind hole 54, tapered end 58 of locking screw 34 pushes against radialwall 44 of fastening screw 32 to urge outward expansion of radial wall44. This outward expansion of radial wall in turn causes outwardexpansion of sidewall 20 of bushing 14, thereby locking fastening screw32, bushing 14, and device 10 to bone as a rigid construct. Because thelocking of fastening screw 32 occurs after fastening screw 32 isthreaded into bone, it is easier to insert fastening screw 32 into bonethan some prior art designs, which result in simultaneous insertion andlocking. Furthermore, because head 38 is physically deformed during thelocking step (as opposed to just physical deformation of the bushing)the assembly according to the present invention improves interlockingbetween bushing 14 and fastening screw 32 to help ensure loosening doesnot occur with time.

While various descriptions of the present invention are described above,it should be understood that the various features can be used singly orin any combination thereof. Therefore, this invention is not to belimited to only the specifically preferred embodiments depicted herein.

Further, it should be understood that variations and modificationswithin the spirit and scope of the invention may occur to those skilledin the art to which the invention pertains. Accordingly, all expedientmodifications readily attainable by one versed in the art from thedisclosure set forth herein that are within the scope and spirit of thepresent invention are to be included as further embodiments of thepresent invention. The scope of the present invention is accordinglydefined as set forth in the appended claims.

What is claimed is:
 1. A method for securing a bone plate having athrough hole to bone, the method comprising the steps of: inserting afastening screw through a bushing located in the through hole of thebone plate; threading the fastening screw into a bone; and locking thefastening screw to the bushing and bone plate after the threading stepis completed.
 2. The method of claim 1 wherein the fastening screw isinserted through the bushing at a surgeon-selected orientation relativeto the bone plate.
 3. The method of claim 1 wherein the fastening screwhas a head and the locking step comprises the step of expanding thefastening screw head against the bushing to thereby lock the bushing tothe bone plate.
 4. The method of claim 3 wherein the expanding stepcomprises the step of inserting a locking screw into a recess in thehead.
 5. The method of claim 1 wherein the fastening screw is aself-tapping screw.
 6. The method of claim 1 wherein the fastening screwis a self-drilling screw.
 7. The method of claim 1 wherein the fasteningscrew has a head portion and a shaft portion and the inserting stepcomprises inserting the shaft portion trough the hole and the headportion is prevented from extending through the hole.
 8. The method ofclaim 7 wherein after the inserting step is complete the shaft portionextends through the hole and the head portion is flush with a topsurface of the bone plate.
 9. The method of claim 1 wherein theinserting step comprises locating the bone plate adjacent to bone andinserting the fastening screw in situ.
 10. A method for securing afixation device having a through hole to bone, the method comprising thesteps of: providing a bushing in the through hole of the fixationdevice; inserting a shaft portion of a bone engaging member into thebushing and the through hole from one side of the fixation device to theother side of the fixation device after the providing step is completed;threading the shaft of the bone engaging member into a bone; and lockingthe bone engaging member to the bushing and fixation device after thethreading step is completed, wherein the bone engaging member has a headand the locking step comprises the step of expanding the bone engagingmember head against the bushing to thereby lock the bushing to thefixation device and wherein the expanding step comprises the step ofinserting a locking screw into a recess in the head.
 11. The method ofclaim 10 wherein the bone engaging member is inserted through thebushing at a surgeon-selected orientation relative to the fixationdevice.
 12. The method of claim 10 wherein the fixation device is a boneplate.
 13. The method of claim 10 wherein the bone engaging member is aself tapping screw.
 14. The method of claim 10 wherein the bone engagingmember is a self-drilling screw.
 15. A method for securing a fixationdevice having a through hole to bone, the method comprising the stepsof: inserting a fastening screw through a bushing located in the throughhole of the fixation device; threading the fastening screw into a bone;and locking the fastening screw to the bushing and fixation device afterthe threading step is completed, wherein the fastening screw has a headand the locking step comprises the step of expanding the fastening screwhead against the bushing to thereby lock the bushing to the fixationdevice.
 16. The method of claim 15 wherein the fastening screw isinserted through the bashing at a surgeon-selected orientation relativeto the fixation device.
 17. The method of claim 15 wherein the expandingstep comprises the step of inserting a locking screw into a recess inthe head.
 18. The method of claim 15 wherein the fixation device is abone plate.
 19. The method of claim 15 wherein to fastening screw is aself-tapping screw.
 20. The method of claim 15 wherein the fasteningscrew is a self-drilling screw.